The present invention relates to ovens; and, more particularly, to apparatus for monitoring and controlling the operation of ovens heated by combustible gases.
Increasing the efficiency of combustible gas-heated ovens is becoming increasingly, economically and ecologically important. The heating requirements for a given oven to properly process a material is largely dictated by the nature of the product and the construction of the oven. Typically, for baking a product based upon a flour-water system, such as crackers or cookies, about 40 to 50% of all heat generated by the combustion gases is required to remove water. Additional heat is required to simply heat the combustion gases, including an excess of about 25% air required for complete combustion, to the temperature of stack discharge. Additional heat is lost with removal of the product and to the surroundings by the processing equipment.
The heating value of the combustion gases is used inefficiently where either too great or too little an amount of heat is supplied. This is true for the oven as a whole and for each individual zone thereof. For example, in the exemplary situation of baking cookies or crackers, supplying heat at too great a rate can actually decrease the effectiveness of the baking operation by causing the product to skin and seal the moisture in the product. Where the amount of heat supplied to the process is less than that required at a particular zone, insufficient heat will be available to drive off water at the optimum rate, and disproportionate amounts of the heat will be lost to factors other than removal of water from the product. And, required adjustments to other zones to correct for the deficiency will result in further inefficiencies.
Automatic temperature control within an oven is advantageous for achieving optimum product quality, but by itself cannot provide oven efficiency. Temperature control does not account for the rate of air input to the oven, the rate of exhaust, the humidity conditions within the oven, the effectiveness of combustion at a particular burner, and the presence of unlit burners, which all play significant roles in proper oven operation. Automatic temperature control was also found to be unsatisfactory due to inter-zone flow which resulted in false indications of temperature in adjacent zones. The result was that steady state conditions could rarely be obtained because the automatic controllers were continuously hunting, making conditions worse than manual pressure control.
Control of the pressure of the incoming combustible gas, in combination with control of the operating temperature, is also often misleading because it does not take into account enough of the significant factors and permits the inefficient use of heat to render the other factors unimportant from the standpoint of product quality. For example, pressure control alone does not inform the oven operator of the existence of burner failure. The unlit burner continues to draw gas which is not utilized for its heating value. The resulting reduced temperature then creates the demand for greater gas flow to that zone as well as adjacent zones with a resulting alteration of temperatures not only in the zone having the unlit burner, but in adjacent zones. Attempting to control oven heat input by controlling the pressure often leads to having the whole oven out of balance since the tendency exists to change adjacent zones to make up the temperature within a target zone.